Imager system and method with filtering

ABSTRACT

An imager apparatus comprises a pixel and a first filter positioned in an incident light path for a portion of the pixel, the filter being operable to alternate between transmitting and reducing incident light on the pixel portion.

BACKGROUND DESCRIPTION OF THE RELATED ART

A variety of consumer and scientific devices use digital imaging andpost-processing to capture and record still and moving images. Flatbedscanners, copy machines, digital cameras, and the Hubbell spacetelescope all use an imager having pixels sensitive to electromagneticradiation (ER) to capture an image of an object. Both CCD(charge-coupled device) and CMOS (complementary-metal oxidesemiconductor) imagers have pixels arranged in either a one ortwo-dimensional array, combined with light-guiding optics. The spatialresolution of an array of pixels in an imager refers to the array'sability to resolve dimensions on a document to be imaged. The resolutioncan be defined as the number of resolvable image units (RIUs)discernable on the imaged object. The spatial resolution of the imagermay be limited by the quality of the mirrors and lens but is fixedbecause of their fixed relative positions in the illustrated system.

Spatial resolution is of particular concern to imager designers, as iscost and light sensitivity, all of which are affected by pixel size. Theresolution of imagers, such as scanners, is fixed, in part, by the sizeof the pixels. If pixel dimensions are halved to allow twice as manypixels in the imager, the spatial resolution of the system could beincreased proportionally. Advances in semiconductor processing haveallowed manufacturers to reduce pixel sizes to allow more pixels perunit area in an imager. Unfortunately, while reducing pixel size allowsimproved spatial resolution, the improvement is at the expense ofreduced signal-to-noise ratios (SNR). The reduction also reduces lightsensitivity and increases susceptibility to pixel blooming caused byimproper shuttering.

Effective shuttering mitigates some of the susceptibility to bloomingcaused by reduced pixel size. Such shuttering may be accomplished eitherelectronically, by varying the frequency of pixel readout and reset (inthe case of a CMOS imager), or mechanically, by controlling the amountof light that reaches the imager. Unfortunately, while use of theshutter helps to resolve blooming issues, it does not help to reduceother problems resulting from smaller pixel size, such as a reduced SNR.

One approach to obtaining better spatial resolution without sacrificingan imager's SNR is to increase pixel size. This approach, however,increases system size and cost. Another approach is to map multiplepoints on an object to the same pixel and perform multiple scans tocapture the points for later recombination by a processor into acomplete image. Separate scans are performed and the images recombinedto form the complete image. Unfortunately, such solutions tend toincrease manufacturing cost and system complexity.

A need continues to exist to improve spatial resolution withoutincreasing system cost, pixel count or reducing pixel size.

SUMMARY

An imager system and method has, in one embodiment, a pixel and a firstfilter positioned in an incident light path for a portion of the pixel,the filter being operable to alternate between transmitting and reducingincident light on the pixel portion.

In one embodiment, a method is described as directing light fromdifferent locations of an object to different portions of the pixel,alternately transmitting and at least partially blocking the light forthe different pixel portions in sequence, and reading out the pixel atdifferent times corresponding to the transmission of the light to thedifferent pixel locations.

BRIEF DESCRIPTION OF THE DRAWINGS

The components in the figures are not necessarily to scale, emphasisinstead being placed upon illustrating the principals of the invention.Moreover, in the figures like reference numerals designate correspondingparts throughout the different views.

FIG. 1 is a block diagram illustrating an embodiment that has a filterarray positioned immediately above an array of pixels, with both arrayscontrolled by a processor.

FIG. 2 is a block diagram illustrating an embodiment with first andsecond filters over the light-sensitive portion of a pixel in a CMOSimager.

FIGS. 3A-3D are perspective views of an embodiment with first throughfourth filters positioned above each individual pixel in an array ofpixels, with each of FIGS. 3A-3D representing different filtering andtransparent states of the filters.

FIG. 4 is a perspective view of an embodiment with odd and evenmultiple-pixel filters, with odd-numbered filters in their filteringstates and even-numbered filters in their transparent states.

FIG. 5 is an exploded perspective view and ray schematic of an opticalshutter, using a ferroelectric liquid crystal (FLC) cell, for use in theembodiments illustrated in FIGS. 3A through 4.

FIG. 6 is a cross section of an embodiment with a filter and pixel arrayseated in a packaging structure.

FIG. 7 is a block diagram and ray schematic of a document scanner thathas a filter array positioned in front of the imager to enhance spatialresolution.

DETAILED DESCRIPTION

An imager, in accordance with an embodiment of the invention, has anarray of ER-sensitive pixels with a plurality of radiation reducingmechanisms (“filters”) associated with portions of each pixel. Althoughthe embodiments are described in terms of ER in the visible spectrum,the wavelength of ER used would depend on the sensitivity of the imagerto that wavelength and the ability of the filters to reduce the ER onthe pixels. Consequently, the term “light” means any ER that the arrayof pixels is sensitive to. The filters are positioned between the pixelsand the object to be imaged. Preferably, the filters are positionedeither immediately adjacent the pixels or immediately adjacent theobject to be imaged. Because the area of each pixel corresponds to aspecific area on the imaged object, the ability of a processor todistinguish between points on the imaged object is increased bysequentially actuating the filters over each pixel in a plurality ofscans which are combined into a single image. In effect, the sequentialactivation and plurality of scans increases the number of pixels in thearray without translating the pixels relative to the imaged objectbetween scans. Embodiments of the invention can be expanded to greaterspatial resolutions by increasing the number of filters associated witheach pixel, and is limited only in the ability to fabricateprogressively smaller filters and to distinguish between progressivelysmaller levels of illumination on each pixel.

FIG. 1 illustrates first and second filters (100, 105) in a filter array110 positioned above individual pixels 115 in a pixel array 120 withboth arrays (110, 120) in communication with a processor 125. Theprocessor 125 is coupled to a timing and control section 130 whichprovides coordinated-control signals to both the pixel array 120 forpixel readout and to vertical and horizontal filter drivers (135, 140)for filter array 110 actuation. The vertical filter driver has first andsecond vertical outputs (136, 138) coupled to each of the first andsecond filters (100, 105), respectively. The horizontal driver has firstthrough fifth horizontal outputs (141, 142, 143, 144, 145) each coupledto a separate first and second filter (100, 105). A particular filter isactuated when it receives both a vertical and horizontal actuationsignal, either positive or negative voltage, from the two drivers (135,140). Each of the pixels 115 provides readout data when they receive areadout control signal from the timing & control section 130.

The readout data is communicated to an analog-to-digital (A/D) converter147 which converts the analog signals to digital data to be saved in amemory 150. The memory 150 is preferably a flash memory, but may includeRAM (random-access memory) or ROM (read-only memory) to accomplish atemporary buffering or permanent storage of readout data. The memory 150is in communication with the processor 125 for image processing and/orretrieval. A user interface 155 communicates with the processor 125 toprovide a user with access to status information such as “power on” and“ready.” The user interface 155 can include a visual display or audiooutput.

The pixel array 120 is responsive to incident light to provide readoutdata and can be, for example, a CCD imager or a CMOS imager having a oneor two-dimensional array of pixels. The processor 125 can be ageneral-purpose digital signal processor (DSP) or an applicationspecific processor (ASIC). If the pixel array 120 is a CMOS imager, theprocessor 125 may be integrated with the pixel array 120 on a singlesubstrate that includes the necessary pixels 115 and timing and controlelement 130. In the embodiment illustrated in FIG. 1, the filter array110 is positioned immediately above the pixel array 120 to reduce straylight introduced between the two. The A/D converter 147 can be asigma-delta or dual slope converter to convert the analog output of thepixel array 110 to a digital form for storage in the memory 150. If thepixel array 110 is a CMOS imager, the A/D converter 145 could beintegrated on a common chip with the array 110 and processor 125.Although the various components are illustrated with electricallyconductive paths between them, other signal transport mechanisms, suchas an optical bus, may also be used.

First and second filters (100, 105) are preferably formed from FLC panesthat become opaque to light upon the application of a positive voltagefrom both the vertical and horizontal filter drivers (135, 140), andtranslucent upon the application of a negative voltage by each (135,140). Alternatively, the filters (100, 105) can be constructed such thatvoltages of an opposite polarity induce the opaque and translucentstates. In either case, for FLC panes, the filter remains opaque ortranslucent after the voltage is removed, and does not change until anopposite polarity voltage is applied. In FIG. 1, the timing and controlsection 130 induces the vertical driver 135 to drive its second verticaloutput 138 with a positive voltage and the first through fifthhorizontal outputs (141-145) of the horizontal filter driver 140 to alsodrive a positive voltage to actuate all of the second filters 105 totheir respective opaque states. If the first filters 100 are not alreadyin their translucent states, the horizontal and vertical filter drivers(140, 135) are induced to drive the first filters 100 with a negativevoltage. Pixel portion B receives substantially reduced incident light,while portion A receives substantially the entire incident light fromthe translucent first filters 100. The timing and control section 130induces a first pixel readout when the second filters 105 are opaque andthe first filters 100 translucent. A second pixel readout is inducedwhen the states of the first and second filters (100, 105) are reversed,so that the processor 125 can distinguish between the spatialcoordinates of an object corresponding to the two sets of filters. Ineffect, the number of pixels is doubled, with each filter correspondingto a separate pixel.

In one embodiment, the first and second filters (100, 105) are notopaque when a positive voltage is applied, but rather semi-opaque tofilter light incident on portions A and B of the pixels 115. With thepredetermined filter opacity known, luminance values would be obtainedfor the two spatial coordinates of the projected image while maintainingbetter light sensitivity than the embodiment described for the opaquefilter states. Also, in a two dimensional implementation, the filterarray would provide filter coverage for each A and B portion of eachpixel in the array. Various techniques can be used to address andcontrol the filters, such as addressing each individual filter insequence, addressing all of the A filters at one time and all of the Bfilters at another time, or grouping the filters by rows, columns orother geometries for addressing.

In a CMOS imager implementation, the first and second filters (100, 105)can be positioned above the entire pixel 115, or above thephotosensitive portions of the pixel. FIG. 2 illustrates first andsecond active-area filters (200, 205) positioned substantially over onlythe photosensitive portion 210 of a pixel manufactured in CMOS (“CMOSpixel 215”). The non-photosensitive portion 220 would contain controlcircuitry that does not respond significantly to incident light andwould not require filtering. For example, in a CMOS imager with a 14%fill-factor (14% of the total pixel area configured to be thephotosensitive portion 210), the first and second active-area filters(200, 205) would each cover at least 7% each of the total pixel area andbe positioned directly above the photosensitive portion 210 of the CMOSpixel 215. The first and second filters (200, 205) can also beimplemented as an array micro-electromechanical elements, filters orshutters that are actuated to block or admit incident light onto thepixel portions A and B.

FIGS. 3A-3D each illustrate an embodiment that has a filter array 300with four filters (305, 310, 315, 320) over portions A, B, C, D,respectively, of each pixel 115. Analogous to the two-filterimplementation shown in FIG. 1, each of the four filters (305, 310, 315,320) are actuated sequentially to correspond with respective sequentialreadouts of the pixels 115. FIG. 3A shows the first filter 305 in atransparent state to communicate incident light 307 to portion A of eachpixel 115. The second, third and fourth filters are placed into anopaque state. The pixels 115 are read out, providing an indication ofthe amount of light 307 incident on portion A. FIGS. 3B, 3C, or 3Dillustrate the processor's 125 actuation of the other filters in thefour-filter sets, which can be programmed to occur in any desiredsequence. As with the two filter per pixel implementation, the filterscan be addressed and actuated separately or in groups with an analogousvertical and horizontal driver set.

FIG. 4 illustrates a perspective view of an embodiment with an array ofmultiple-pixel filters 400, with each filter positioned in the incidentlight path 402 for two adjacent pixels in an array of pixels 120. Asillustrated, “odd” and “even” multiple-pixel filters (405, 410) are inthe incident light paths for B and A portions, respectively, of thepixels 115. Thus, unlike the filters illustrated in FIGS. 3-5D, whereeach filter is associated with only one pixel, the multiple-pixel filterarray 400 illustrated in FIG. 4 is configured for each filter toassociate with equal halves of two pixels 115. Analogous to thetwo-filter implementation shown in FIG. 1, each of the illustrated oddand even multiple-pixel filters (405, 410) are placed sequentially intheir filtering and transparent states during respective pixel readoutsto provide an indication of the amount of light 402 incident on A and Bportions. Embodiments of the invention can be expanded for atwo-dimensional array, with a single filter covering a portion ofmultiple pixels to distinguish between the amount of incident light oneach portion of each pixel.

FIG. 5 illustrates an exploded perspective view of an implementation offirst and second filters (100, 105) with the first filter 100 in itsopaque state and the second 105 in its transparent state. Each filter(100, 105) includes respective thin FLC layers (500, 502) positionedbetween respective incoming (505, 507) and cross (510, 512) polarizers.Each of the cross polarizers (510, 512) are optically rotated by +90degrees from the initial polarizers (505, 507). In their original state,the FLC layers (500, 502) have their optical axis rotated +45 degreesfrom the optical axis of their respective initial polarizers (505, 507).

In the first filter 100, the optical axis of the FLC layer 500 isrotated by +45 degrees upon application of a positive DC voltage toalign the layer 500 with the optical axis of the initial polarizer 505.Non-polarized light 515 introduced to the initial polarizer 505 becomesvertically polarized and the FLC layer 500 does not change theorientation of the polarization axis of the transmitted light.Consequently, the first filter's 100 crossed polarizer 510 blocks thepolarized light 520 and the filter is considered to be in its opaquestate.

The second filter 105 is illustrated with the optical axis of its FLClayer 502 rotated by +45 degrees from its original +45 degree rotationupon application of a negative DC voltage, so that the layer 502 isaligned with the optical axis of its cross polarizer 512. Lightpolarized by the first polarizer 507 is rotated +90 degrees by the FLC502 to allow the light to pass through the second polarizer 510.

FIG. 6 illustrates an embodiment with the filter array positioneddirectly on the pixel array. The cross section illustrates the filterand pixel arrays (110, 120) seated in a rigid package 600 that could beformed from rigid plastic, metal, or composite molding to hold thearrays (110, 120) and protect them from damage.

FIG. 7 illustrates an embodiment of the invention in a document scanner.An object 700 is illuminated with a light source 705, and the object'simage is directed to scanner carriage 707 that has a light director 725that, in one implementation, is a lens to focus the image onto the pixelarray 120 through the filter array 110. Various light director schemescan be used to direct the image onto the pixel array, depending upon thephysical arrangement of the system components. In the illustration ofFIG. 7, three mirrors 710, 715 and 720 and a lens 725 are used in thescanner carriage 707. For CMOS and CCD imagers, the light source 705 canproduce florescent or incandescent ER in the visible spectrum (380through 780 nanometers). The lens 725 is a converging lens to reduce theimage onto the filter and pixel arrays (110, 120). The lens 725 mayinclude coatings to reduce the transmission of stray or reflected lightto the pixel array 120. The carriage 707 is translated relative to theobject 700 during multiple scans to capture an image of the entireobject 700 in image post-processing.

Instead of positioning a filter array on the imager, a filter array 730may be positioned immediately adjacent the object 700 to be imaged. Eachpixel mapped onto the object (the scanner platen) would be associatedwith at least two adjacent filters on the filter array 730. Thereflection of light 705 off the filter array 730 would be removed inimage post-processing to leave only the light reflected from thenon-filtered mapped spaces on the object 700. For example, a sampleimage would be taken to capture the reflection of light off of allfilters in their opaque states. This sample image would then besubtracted from an object image in post-processing to complete a trueimage of the imaged object. Various filter sequencings can be used, asdescribed above.

While various embodiments of the invention have been described, it willbe apparent to those of ordinary skill in the art that many moreembodiments and implementations are possible. For example, while animager system has been described as including a light source toilluminate an object, the invention is also applicable to the imaging ofobject that emits its own radiation, as in infrared imaging.

1. An imager apparatus, comprising: a pixel; and a first filterpositioned in an incident light path for a portion of said pixel, saidfilter being operable to alternate between transmitting and reducingincident light on said pixel portion.
 2. The imager apparatus of claim1, wherein said filter is actuable to block substantially all incidentlight on said pixel portion.
 3. The imager apparatus of claim 2, whereinsaid filter comprises a ferroelectric liquid crystal filter.
 4. Theimager apparatus of claim 1, further comprising: at least one additionalfilter positioned in an incident light path for a respective portion ofsaid pixel and being operable to alternate between transmitting andreducing incident light on said respective portion of said pixel.
 5. Theimager apparatus of claim 4, wherein each additional filter is actuableto block substantially all incident light to respective pixel portions.6. An imager system, comprising: an array of pixels; a light directorpositioned to direct light from an object onto said pixels; a pluralityof filters associated with each of said pixels, each of said filtersbeing operable to alternate between transmitting and filtering saidlight for a respective portion of its associated pixel that is differentfrom the pixel portion for each other filter associated with the samepixel; and an operating system connected to operate the filtersassociated with each pixel in sequence to provide an image spatialresolution greater than without said filters.
 7. The imager system ofclaim 6, wherein said filters are operable to block substantially alllight from their respective pixel portions.
 8. The imager system ofclaim 6, wherein said filters are operable to block substantially lessthan all light from their respective pixel portions.
 9. The imagersystem of claim 6, wherein said operating system operates the filters ofeach pixel separately from the filters of the other pixels.
 10. Theimager system of claim 6, wherein said operating system concurrentlyoperates the filters in groups of pixels.
 11. The imager system of claim6, wherein said filters are positioned adjacent their respective pixels.12. The imaging system of claim 6, wherein said filters are positionedadjacent an object to be imaged.
 13. A resolution enhancement method,comprising: filtering incident light from an image to a first portion ofa pixel in an imager; reading out a first light indication from saidpixel; filtering incident light from the image to a second portion ofsaid pixel; and reading out a second light indication from said pixel sothat said pixel can distinguish between two spatial regions on saidimage.
 14. The method of claim 13, wherein said filtering comprisessubstantially blocking said light.
 15. Method for scanning an object,comprising: directing light from different locations of the object todifferent portions of a pixel; alternately transmitting and at leastpartially blocking said light for said different pixel locations insequence; and reading out said pixel at different times corresponding tothe transmission of said light to said different pixel portions.
 16. Themethod of claim 15, wherein said pixel is in an array of pixels, furthercomprising directing light from different respective locations of saidobject to different portions of each of said pixels, alternatelytransmitting and at least partially blocking said light for saiddifferent pixel portions in sequence, and reading out said pixels sothat each pixel can distinguish between more than one spatial region onsaid image.
 17. The method of claim 16, wherein said light issubstantially fully blocked for said different pixel locations.
 18. Themethod of claim 16, wherein said light is alternately transmitted and atleast partially blocked for each of said pixels separately.
 19. Themethod of claim 16, wherein said light is alternately transmitted and atleast partially blocked for groups of said pixels concurrently.